The cellular slime molds, Dictyostelium discoideum and other species consist of independent amoeboid cells during the growth phase. At the completion of growth the cells respond to cyclic AMP and other chemotactic substances by aggregating to form multicellular pseudoplasmodia. Following pseudoplasmodium formation the cells differentiate into prestalk and prespore cells which subsequently compose a mature sorocarp. The proposed research involves a study of the role of Contact A and B sites on cell differentiation in D. discoideum. The Contact A and B sites are antigenic and specific antisera (Fab) has been shown to block contacts between differentiation. Consequently the effect of blocking either Contact A or B sites independently or simultaneously with Fab may provide evidence that one or both of the sites are necessary to effect cell differentiation. The question of whether the cells differentiate when wholly or partially blocked by Fab will be determined by the pattern of (3H) fucose observed in autoradiographs and by EM examination for the presence of marker organelles the prespore vacuoles. An additional study will be conducted upon the effect of breaking cell contacts in various regions of the migrating pseudoplasmodium. This technique results in redifferentiation of one cell type to another in spatial patterns dependent upon the region in which the cell contacts are broken. This study is expected to yield evidence concerning the nature of the signals which initiate the redifferentiation process.